It is a repeating motif in transporter use that the transporter operator and the machine itself are ignorant of what they are transporting: stowaways can "enter the beam" easily enough, say. So we would do wisely to assume that the phasing effect of the transporter is naturally terminated at the edge of a thing being transported - that is, at the point where air or vacuum ends and something solid begins. The machine doesn't need to care: the effect just stops propagating because it hits a "potential barrier".

The same should work for the phaser effect, which may indeed be the exact same thing as the transporter effect. Once a phaser beam or bolt hits a spot, it spreads out its make-disappear effect until it hits a "phase border" into a much denser or much less dense material. Say, it removes the steel kettle but leaves the interior dough and the exterior air when Valeris fires in ST6:TUC.

Hopping of the make-disappear effect from a person to his clothes (or vice versa) happens in every instance we witness, which is quite understandable because clothes aren't all that different from people (in TAS, they even have a line saying the uniforms are algae-based). Hopping to handheld objects also happens, which means the effect can clear not-insignificant "potential barriers" into what looks like metal - Klingon guns or swords don't clatter to the ground when the warriors disappear into wherever (and it doesn't much matter whether this is due to the attacker just using a higher setting to clear the "potential barrier" - it suffices to observe that this happens). So why no hop from the victim to the floor or the ground?

We may easily argue that starship floors are phaserproof by design, and present a sharp "phase border" to the effect. After all, random phaser blasts never harm ship floors, walls or ceilings. But why would random dirt be different from the constituent matter of a Klingon? Perhaps there is some minor dissipating of the sand or grass below a phaser victim (or a transportee!) - the mystery lies in why this should be invisible, when the removal or cutting of of grass typically is easy to spot by the Mk I Eyeball.

A couple of episodes feature the heroes doing forensics on a fight site to determine whether phasering or transporting took place. Perhaps their tricorders indeed notify them of a slight dissipation of the floor?